1. Field of the Invention
This invention relates to an intake manifold structure for a V-type engine.
2. Description of the Prior Art
Recently there have been made various attempts to effectively utilize the inertia effect of engine intake air in order to improve the volumetric efficiency, thereby improving the output power of an engine. The inertia effect of intake air is enhanced with the reduction in the flow resistance in the intake passage and the increase in the length of the intake passage. Accordingly, in order to sufficiently improve the volumetric efficiency by the inertia effect of intake air, it is required that the intake manifold is as straight and long as possible.
In the case of an in-line engine, a relatively large space remains in the engine compartment at the opposite sides of the engine, and accordingly, an intake manifold which can satisfy such a requirement can be easily mounted. On the other hand, in the case of a V-type engine, a pair of cylinder banks are disposed in the engine compartment spaced transversely from each other, and accordingly, the space on the opposite sides of the engine is relatively small and is not sufficient to accommodate an intake manifold which can satisfy the requirement described above.
Therefore, in the V-type engine, the intake manifolds for the right and left cylinder banks are arranged to cross each other and are disposed above the space between the cylinder banks in order to satisfy the requirement described above as disclosed, for instance, in Japanese Unexamined Patent Publication No. 59(1984)-115460.
It is preferred that the intake manifolds for the right and left cylinder banks be integrally formed in order to facilitate incorporation of the intake manifolds in the engine. Therefore, conventionally, the intake manifolds are generally formed in one piece by cast molding.
However, in the case that the integral intake manifolds are molded by molds having no core, many undercut portions are required in the molds since the parting line of the molds is complex, and accordingly the number of post-processing steps is increased, canceling out the advantage of cast molding. On the other hand, when the integral intake manifolds are molded by molds having cores for forming the undercut portions, the molding operation itself is complicated, making mass production thereof difficult, thereby adding to the manufacturing cost.